1. Field of the Invention
The present invention generally relates to reproducible color gamut compression apparatuses and reproducible color gamut compression methods and, more particularly, to a method and apparatus of reproducible color gamut compression, adapted for a combination of a plurality of color display devices such as monitors that are different in reproducible color gamut, for converting a color that cannot be reproduced by a target display device into a reproducible color. More particularly, the invention relates to a reproducible color gamut compression method and a reproducible color gamut compression apparatus, in which the reproducible color gamut is compressed depending on a characteristic factor, the volume of computation is small and the processing speed is relatively high.
2. Description of the Related Art
For example, Japanese Laid-Open Patent Application No. 9-9829discloses a reproducible color gamut compression method designed to meet such demands. Japanese Laid-Open Patent Application 9-98298 proposes a reproducible color gamut compression method and a reproducible color gamut compression apparatus adapted for color DTP (Desk Top Publishing) wherein a consideration is given to differences between display devices in reproducible color gamut so that natural color reproduction is effected. The apparatus and method disclosed in Japanese Laid-Open Patent Application 9-98298 realize smooth reproducible color gamut compression for an image characterized by a relatively high color saturation, by using different compression methods for different areas.
FIG. 8 is schematic view of a reproducible color gamut compression process according to the reproducible color gamut compression method disclosed in Japanese Laid-Open Patent Application 9-98298. The color reproduction gamut compression method is such that an area outside the color gamut, which is reproducible by a monitor but not reproducible by a printer, is divided into three areas so that compression occurs in the respective areas.
A description will now be given of how the reproducible color gamut compression method is performed.
Given that a reproducible color gamut GMout of an output device is smaller than a reproducible color gamut GMin of an input device, and a hue h is constant, two straight lines are drawn in a plane defined by an axis of brightness L* and an axis of color saturation C* so as to divide the reproducible color gamut GMin of the input device is into four areas including a common area AR1, a high-brightness area AR2, a low-brightness area AR3 and a high color saturation area AR4. Color gamut compression is performed for each of the areas AR1-AR4, the direction of compression being different from area to area. In this manner, a color residing in the reproducible color gamut GMin of the input system is converted into a color residing in the reproducible color gamut GMout of the output device.
A color residing in the common area AR1 shared by the input reproducible color gamut GMin and the output reproducible color gamut GMout is not subject to conversion. The high-brightness area AR2 is subject to compression toward (0, L*_min), and the high color saturation area AR4 is subject to compression toward (C*_th, L*th). L*_min indicates a minimum value of brightness of the output reproducible color gamut GMout and L*_max indicates a maximum value of brightness of the input reproducible color gamut GMin and the output reproducible color gamut GMout, C*max, L*_th indicate a maximum value of color saturation in the output reproducible color gamut GMout and an associated brightness, respectively. C*_th=C*_max×K, where K is a positive number smaller than 1.
By using the related-art color gamut compression method, i.e., by drawing two straight lines on a plane defined by brightness and color saturation under a condition that the reproducible color gamut GMout of the output device is smaller in area than the reproducible color gamut GMin of the input device and that the hue is constant, dividing the input reproducible color gamut GMin into four areas by the two straight lines, performing color gamut compression in directions different from area to area so that a color residing in the input reproducible color gamut GMin is converted into a color residing in the output reproducible color gamut GMout, the color gamut compression ensures natural appearance of colors.
One disadvantage with the color gamut compression method according to the related art is that the directions of compression differ from area to area in a discontinuous manner so that the smooth transition between colors cannot be ensured at the borders between adjacent areas. The related-art color gamut compression is capable of compression wherein vividness of an image with high color saturation is maintained. In contrast, however, a color in the high-brightness area AR2 is converted into a less vivid color and a color in the low-brightness area AR3 is converted into a color characterized by upward level shift of blackness. Upward level shift of blackness does not present a problem in a combination of a monitor and a printer because the capability of a printer to reproduce gradation in the low-brightness area AR3 is inherently low. However, upward level shift of blackness presents a problem in a combination of monitors. Thus, the visible effect produced by the related-art color gamut compression is different from image to image. If an image is characterized by a small low-brightness area AR3, the related-art method may be favorably applied, causing no problem of upward level shift of blackness. If an image is characterized by a large high color saturation area AR4, a significant favorable effect is provided. Another disadvantage with the related-art color gamut compression is that it takes a relatively long period of time for color space conversion since the color gamut compression is performed in the LAB color space. Real-time compression is difficult and the method is not suitably used for a display device such as a monitor.
Similar color gamut compression methods are disclosed in Japanese Laid-Open Patent Application No. 11-341296, Japanese Laid-Open Patent Application No. 4-284579, Japanese Laid-Open Patent Application No. 9-9087, Japanese Laid-Open Patent Application No. 9-74494, Japanese Laid-Open Patent Application No. 9-168097, Japanese Laid-Open Patent Application No. 11-141209, Japanese Laid-Open Patent Application No. 10-200764, Japanese Laid-Open Patent Application No. 10-229500, Japanese Laid-Open Patent Application No. 10-210275, Japanese Laid-Open Patent Application No. 11-275377, and Japanese Laid-Open Patent Application No. 11-69189.
As described, since the related-art color gamut compression method involves a problem in that upward level shift of blackness may be produced when a color in the low-brightness area AR3 is subject to compression. Upward level shift of blackness does not present a problem if a target display device is a printer. But it does present a problem in a monitor. The result of color gamut compression according to the related art depends heavily on the characteristic of image. If every image is subject to the same process of color gamut compression, it is not ensured that resulting images successfully pass predetermined criteria of evaluation. Since the continuity between the areas AR1–AR4 is poor and the color gamut compression is performed in the LAB color space, it takes a relatively long period of time to perform color space conversion, thus making it difficult to process compression on a real time basis. Accordingly, the related-art method is not suitable for a monitor.